1. Field of the Invention
The present invention relates to a static random access memory (SRAM), and for example, a static noise margin (SNM) of a memory cell.
2. Description of the Related Art
The memory cells of an SRAM typically have first and second inverters whose inputs and outputs are connected to each other, and first and second transfer transistors that connect the output ends of the first and second inverters to a bit line pair. The first and second inverters include a load transistor and a driver transistor. Gates of the first and second transfer transistors are connected to a word line WL.
Conventionally, in order to measure SNM, measurement memory cells which are provided separately from normal memory cells are used. The measurement memory cells have a pad connected to respective nodes in addition to the constitution of the memory cells. At the time of measurement, a bit line and a word line are fixed at a potential VDD. In a state that terminals of the first and second inverters are biased to a predetermined potential, a relationship (transfer curve) between nodes NB and NA corresponding to an input terminal and an output terminal of the first inverter is obtained. That is to say, a potential supply end to the second inverter is opened and a potential supply ends to the first inverter are biased to potentials VDD and VSS. In this state, the potential of the node NB changes from VSS to VDD. When the potential of the terminal NB is plotted along the abscissa and the potential of the terminal NA is plotted along the ordinate, the transfer curve of the first inverter is obtained.
Similarly, in a state that the potential supply end to the first inverter is opened and the potential supply ends to the second inverter are biased to potential VDD and potential VSS, the potential of the node NA changes from VSS to VDD, so that a transfer curve of the second inverter is obtained. The two transfer curves are overlapped. As a result, the stability characteristics of a cell (butterfly curve) can be obtained as shown in FIG. 1 in the article “The Impact of Intrinsic Device Fluctuations on CMOS SRAM Cell stability” by Azzez j. Bhavnagarwala et al., in IEEE journal Of Solid-state Circuits”, April, 2001, Vol. 36, No. 4, p. 659. The length of one side of a square inscribed in two areas surrounded by the two transfer curves corresponds to SNM. In general, the larger the SNM, the harder it is for data destruction to occur because of power-source voltage noise in a chip. For this reason, it is important when designing memory cells to set SNM to a large value, and an SNM check is important for the development of cells.
The above method, however, enables only the measurement of SNM in the measurement memory cells, but cannot measure SNM in memory cells on chips as products. Further, since an actual measurement is made in a state that pins are attached to respective pads in each measurement cell, a very long measurement time is required, and the amount of data obtained is limited.